Hydraulically driven vehicle

ABSTRACT

The present invention includes a hydraulic pump ( 11 ), a variable displacement hydraulic motor ( 5 ) for traveling driven by pressure oil from the hydraulic pump ( 11 ), a motor displacement control means ( 17, 18 ) for adjusting a displacement of the hydraulic motor ( 5 ) in correspondence to a drive pressure at the hydraulic motor ( 5 ), an operating member ( 22 ) through which a forward travel command and a backward travel command for the vehicle are issued, a control means ( 12 ) to be driven in response to an operation of the operating member ( 22 ), for controlling a flow of pressure oil from the hydraulic pump ( 11 ) to the hydraulic motor ( 5 ), a reverse operation detection means ( 41 A,  41 B) for detecting a reverse operation of the operating member ( 22 ) performed to a reverse side opposite from a direction along which the vehicle is advancing, and a cavitation preventing means ( 25 A, 25 B) engaged in operation so as to prevent occurrence of cavitation at the hydraulic motor ( 5 ) when the reverse operation at the operating member ( 22 ) is detected by the reverse operation detection means ( 41 A,  41 B).

TECHNICAL FIELD

The present invention relates to a hydraulically driven vehicle such asa wheel hydraulic excavator having a variable displacement travelingmotor.

BACKGROUND ART

In a wheel hydraulic excavator having a variable displacement travelingmotor in the related art, the motor displacement is controlled bydriving a motor regulator in correspondence to the drive pressure at thetraveling motor. In such a wheel hydraulic excavator, the motordisplacement increases as the drive pressure rises to drive the motor atlow speed with high torque and the motor displacement decreases as thedrive pressure becomes lower to drive the motor at high speed with lowtorque.

More specifically, the motor displacement is fixed at a constant value(e.g., the minimum displacement) over a predetermined low motor drivepressure range so as to minimize the extent to which the traveling speedchanges due to fluctuations in the motor drive pressure when travelingon flat ground or downhill, whereas the motor displacement is increasedas the motor drive pressure increases beyond the predetermined range soas to increase the rotation torque at the motor during acceleration orwhen the vehicle is traveling uphill.

As either the front side (toward the toes) or the rear side (toward theheel) of the accelerator pedal in a wheel hydraulic excavator isdepressed, the accelerator pedal is allowed to rotate along theforward/rearward direction. As the front side or the rear side of theaccelerator pedal is depressed, the control valve is switched from theneutral position to a forward travel position or a reverse travelposition, and pressure oil is supplied from a hydraulic pump to thetraveling motor to generate a motor drive pressure. As the acceleratorpedal is released while traveling, the control valve is switched to theneutral position, thereby cutting off the pressure oil supply from thehydraulic pump to the traveling motor. Subsequently, the vehicle travelswith an inertial force and the function of the traveling motor isswitched from the motor function to a pump function. The motor drivepressure decreases at this time and if the vehicle has been driven witha large motor displacement, the motor displacement is reduced, whereasif the vehicle has been driven with the minimum displacement, theminimum motor displacement is sustained. As a result, the quantity ofoil intake required to rotate the traveling motor decreases to inhibitthe occurrence of cavitation.

However, if a reverse operation is performed at the accelerator pedalwhile the vehicle is traveling, i.e., if the rear side of theaccelerator pedal is depressed while the vehicle is traveling forward,the control valve is switched to the reverse travel position and, as aresult, the motor drive pressure increases in a state in which thevehicle is traveling with an inertial force. Consequently, the motordisplacement increases to lead to an increase in the quantity of oilintake required for traveling motor rotation, giving rise to the risk ofcavitation.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a hydraulically drivenvehicle capable of preventing the occurrence of cavitaion when a reverseoperation is performed at an accelerator pedal.

A hydraulically driven vehicle according to the present inventionincludes a hydraulic pump, a variable displacement hydraulic motor fortraveling driven by pressure oil from the hydraulic pump, a motordisplacement control means for adjusting a displacement of the hydraulicmotor in correspondence to a drive pressure at the hydraulic motor, anoperating member through which a forward travel command and a backwardtravel command for the vehicle are issued, a control means to be drivenin response to an operation of the operating member, for controlling aflow of pressure oil from the hydraulic pump to the hydraulic motor, areverse operation detection means for detecting a reverse operation ofthe operating member performed to a reverse side opposite from adirection along which the vehicle is advancing, and a cavitationpreventing means engaged in operation so as to prevent occurrence ofcavitation at the hydraulic motor when the reverse operation at theoperating member is detected by the reverse operation detection means.

In this manner, the occurrence of cavitation can be prevented when anaccelerator pedal is pressed in a direction opposite to a direction inwhich a vehicle is advancing.

The cavitation preventing means may be configured to inhibit an increasein the displacement of the hydraulic motor. It may be configured toblock an operation signal from the operating member. It may also beconfigured to cut off the flow of pressure oil from the hydraulic pumpto the hydraulic motor. The drive pressure may instead be reduced.

It is preferable to prevent occurrence of cavitation when the rotationspeed of the hydraulic motor exceeds a reference value and the reverseoperation at the operating member is detected. It is possible to preventcavitation in accordance with a vehicle speed.

In this case, a reference value of the motor rotation speed may be setsmaller as an inertial force applied to the vehicle becomes greater. Agrade of a road surface or vehicle weight may be detected so as todetect the inertial force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents an external view of a wheel hydraulic excavator in whichthe present invention is adopted;

FIG. 2 is a circuit diagram of a traveling hydraulic circuit in thehydraulically driven vehicle achieved in a first embodiment;

FIG. 3 is a block diagram of a control circuit that controls solenoidcontrolled directional control valves in FIG. 2;

FIG. 4 presents a flowchart of an example of processing that may beexecuted in a controller shown in FIG. 3;

FIG. 5 is a circuit diagram of a traveling hydraulic circuit in thehydraulically driven vehicle achieved in a second embodiment;

FIG. 6 is a circuit diagram of a traveling hydraulic circuit in thehydraulically driven vehicle achieved in a third embodiment;

FIG. 7 is a block diagram of a control circuit that controls aforward/backward switching valve in FIG. 6;

FIG. 8 presents a flowchart of an example of processing that may beexecuted in a controller shown in FIG. 7;

FIG. 9 is a circuit diagram of a traveling hydraulic circuit in thehydraulically driven vehicle achieved in a fourth embodiment;

FIG. 10 is a block diagram of a control circuit that controls a solenoidcontrolled directional control valve in FIG. 9;

FIG. 11 presents a flowchart of an example of processing that may beexecuted in a controller shown in FIG. 10;

FIG. 12 shows an example of the reference rotation speed setting thatmay be adopted when switching the solenoid controlled directionalcontrol valve; and

FIG. 13 shows another example of the reference rotation speed settingthat may be adopted when switching the solenoid controlled directionalcontrol valve.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

The following is an explanation of a first embodiment achieved byadopting the present invention in a wheel hydraulic excavator, given inreference to FIGS. 1 to 4.

As shown in FIG. 1, the wheel hydraulic excavator includes anundercarriage 1 and a revolving upperstructure 2 rotatably mounted atopthe traveling superstructure 1. An operator's cab 3 and a work frontattachment 4 constituted with a boom 4 a, an arm 4 b and a bucket 4 care disposed at the revolving upperstructure 2. The boom 4 a is hoistedas a boom cylinder 4 d is driven, the arm 4 b is hoisted as an armcylinder 4 e is driven and the bucket 4 c is engaged in a lift operationor a dump operation as a bucket cylinder 4 f is driven. A variabledisplacement hydraulic traveling motor 5, which is hydraulically driven,is installed at the traveling superstructure 1.

FIG. 2 is a circuit diagram of a traveling hydraulic circuit in thewheel hydraulic excavator. As shown in FIG. 2, oil discharged from avariable displacement main pump 11 driven by an engine 10, the directionand the flow rate of which are controlled with a control valve 12, issupplied to a variable displacement traveling motor 5 via a brake valve14 having a built-in counterbalance valve 13. The rotating speed of thetraveling motor 5 is changed by a transmission 7 capable of changing thegear ratio over, for instance, two stages, i.e., low and high. Followingthe gear change, the rotation is transmitted to tires 6 via a driveshaft 8 and axles 9, thereby engaging the wheel hydraulic excavator inthe traveling motion.

The displacement angle of the main pump 11 is adjusted with a pumpregulator 11A. The pump regulator 11A includes a torque limiting unit towhich the pump discharge pressure is fed back to enable horsepowercontrol. The term “horsepower control” in this context refers to controlof the pump displacement implemented so as to ensure that the loaddetermined in correspondence to the pump discharge pressure and the pumpdisplacement does not exceed the engine output. In addition, theregulator 11A includes a maximum displacement limiting unit thatdetermines the maximum flow rate at the main pump 11.

The direction in which the control valve 12 is switched and the strokequantity at the control valve 12 are controlled in conformance to atraveling pilot pressure supplied from a pilot circuit, and by adjustingthe stroke quantity, the traveling speed of the vehicle can becontrolled. The pilot circuit includes a pilot pump 21, a pair oftraveling pilot valves 23A and 23B that generate a secondary pilotpressure as an accelerator pedal 22 is depressed, a pair of slow returnvalves 24A and 24B connected at a stage at the rear of the pilot valves23A and 23B respectively to slow the oil returning to the pilot valves23A and 23B and a pair of solenoid controlled directional control valves25A and 25B that allow or prohibit generation of a traveling pilotpressure.

As the front side of the accelerator pedal 22 is depressed (front pressoperation) or the rear side of the accelerator pedal 22 is depressed(rear press operation), the accelerator pedal 22 is allowed to rotatealong the forward direction or the rearward direction. In response to afront press operation at the accelerator pedal 22, the pilot valve 23Ais driven, whereas the pilot valve 23B is driven in response to a rearpress operation at the accelerator pedal 22. As the pilot valve 23A or23B is driven, a pilot pressure achieving a level corresponding to theextent to which the accelerator pedal 22 has been operated is generated.This pilot pressure is detected at a pressure sensor 41A or 41B as anoperation signal Pf or Pr from the accelerator pedal 22.

A governor of the engine 10 is connected to a pulse motor (not shown)and as the pulse motor rotates, the governor is driven. The rotation ofthe pulse motor is controlled in correspondence to the extent to whichthe accelerator pedal 22 is operated. Thus, as the accelerator pedal 22is operated to a greater extent, the engine rotation speed increases,and as the accelerator pedal 22 is operated to a lesser extent, theengine rotation speed becomes lower. After the accelerator pedal 22 isreleased, the engine rotating rate shifts to an idling speed. It is tobe noted that the engine may rotate at a constant rotation speedregardless of the extent to which the accelerator pedal 22 is operated.

The traveling motor 5 includes a device with automatic displacementcontrol by drive pressure related and its displacement is increased asthe drive pressure rises so as to drive it at low speed with hightorque, whereas its displacement is decreased as the drive pressurebecomes lower, so as to drive it at high speed with low torque. It is tobe noted that the motor displacement remains unchanged even if the motordrive pressure fluctuates over a predetermined relatively low motordrive pressure range to sustain the minimum displacement, and that oncethe motor drive pressure increases to a level beyond the predeterminedrange, the motor displacement is increased in correspondence to theincrease in the drive pressure. As a result, when the vehicle travels onflat ground or downhill with the motor drive pressure at a relativelylow level, a change in the traveling speed due to fluctuations in themotor drive pressure is minimized, but high torque is achieved duringacceleration or when the vehicle travels uphill with the motor drivepressure at a high level. The drive pressure is applied to a controlpiston 17 and a servo piston 18 of the traveling motor 5 from a shuttlevalve 16 within the brake valve 14.

If a front step operation, for instance, is performed at the acceleratorpedal 22 while the solenoid controlled directional control valves 25Aand 25B are both at a position “a” as shown in the figure, the pilotpressure from the main pump 21 is applied to one of pilot ports at thecontrol valve 12, and the control valve 12 is switched to an F positionby the pilot pressure. With the control valve 12 thus switched, the oildischarged from the main pump 11 is guided to the traveling motor 5 viathe control valve 12, a center joint 15 and the brake valve 14, and isalso applied to the counterbalance valve 13 as a pilot pressure, therebyswitching the counterbalance valve 13 from the neutral position. As aresult, the traveling motor 5 is driven to engage the wheel hydraulicexcavator in a forward traveling motion.

If the operation of the accelerator pedal 22 ceases in this state, thepressure oil from the pilot pump 21 is cut off by the pilot valve 23Aand an outlet port comes into communication with a reservoir. As aresult, the pressure oil having been applied to the pilot port at thecontrol valve 12 is caused to return to the reservoir via the slowreturn valve 24A and the pilot valve 23A. Since the returning oil iscompressed at a restrictor of the slow return valve 24A, the controlvalve 12 is gradually switched to the neutral position. Once the controlvalve 12 is switched to the neutral position, the oil discharged fromthe main pump 11 is allowed to return to the reservoir, the supply ofthe drive pressure oil to the traveling motor 5 is cut off and thecounterbalance valve 13, too, is switched to the neutral position in thefigure.

In this state, the vehicle body continues traveling with an inertialforce and the traveling motor 5 switches from the motor function to apump function, with a B port and an A port in the figure constitutingthe intake side and the outlet side respectively. Since the pressure oilfrom the traveling motor 5 is compressed at a restrictor (neutralrestrictor) at the neutral position of the counterbalance valve 13, thepressure between the counterbalance valve 13 and the traveling motor 5rises and works on the traveling motor 5 as a braking pressure. Inresponse, a braking torque is generated at the traveling motor 5 toapply brakes on the vehicle body. In addition, since the drive pressure(the pressure on the B port side) at the traveling motor 5 decreases inthis situation, the motor displacement becomes smaller unless thedisplacement of the motor 5 has already been at the minimum displacementlevel, and the minimum displacement is sustained if the motordisplacement has already been at the minimum displacement level.Consequently, the quantity of oil intake needed to rotate the travelingmotor 5, too, becomes smaller. If the quantity of oil intake becomesinsufficient while the motor is functioning as a pump, supplementary oilis supplied to the traveling motor 5 from a makeup port 19. The maximumbraking pressure is regulated with relief valves 20A and 20B. Thereturning oil at the relief valves 20A and 20B is guided to the intakeside of the traveling motor 5.

In contrast, if a rear step operation (reverse operation) is performedat the accelerator pedal 22 while the vehicle is traveling in responseto a front step operation at the accelerator pedal 22, the travelingmotor 5 is driven with the inertial force of the vehicle body and thefunction of the traveling motor 5 switches from the motor function tothe pump function in a manner similar to that described above. As thecontrol valve 12 is switched to an R position in response to the reverseoperation at the accelerator pedal 22, the drive pressure generated onthe A port side by the main pump 11 switches the counterbalance valve 13to the right side position in the figure, and the pressure in the Aport-side pipeline is caused to rise drastically by the pressure oildischarged from the traveling motor 5 acting as a pump and the pressureoil discharged from the main pump 11. Thus, the pressure in the Aport-side pipeline (motor drive pressure) increases in response to thereverse operation at the accelerator pedal 22, the high pressure oil isguided by the shuttle valve 16 to the pistons 17 and 18 to increase themotor displacement, and the quantity of oil intake needed to rotate thetraveling motor 5, too, increases. This gives rise to a concern for theoccurrence of cavitation due to insufficient oil supplement to thetraveling motor 5. In order to prevent the occurrence of cavitation, thesolenoid controlled directional control valves 25A and 25B arecontrolled in the embodiment as described below.

FIG. 3 is a block diagram of a control circuit that controls thesolenoid controlled directional control valves 25A and 25B. The pressureswitches 41A and 41B and a rotation speed sensor 42 that detects therotation speed of the traveling motor 5 are connected to the controller40 constituted with a CPU and the like. Specific processing is executedin the controller 40 based upon signals input thereto from the pressureswitches and the rotation speed sensor and the controller 40 thenoutputs control signals for the solenoid controlled directional controlvalves 25A and 25B. In addition, a neutral switch 43 is connected to thesolenoid controlled directional control valves 25A and 25B. As theneutral switch 43 is turned on, the solenoid controlled directionalcontrol valves 25A and 25B are each switched to a “b” position,regardless of the control signal output from the controller 40.

FIG. 4 presents a flowchart of an example of processing that may beexecuted in the controller 40. In step S1, a decision is made as towhether or not the rotation speed N of the traveling motor 5 detectedwith the rotation speed sensor 42 is equal to or lower than apredetermined reference rotation speed N1. This decision is made tojudge the likelihood of cavitation. Namely, since the inertial force ofthe vehicle increases as the rotation speed of the traveling motor 5becomes high, a larger quantity of oil needs to be taken into thetraveling motor 5 following a reverse operation at the accelerator pedal22, giving rise to an increased concern for cavitation. Accordingly, arotation speed of the traveling motor 5 at which the concern forcavitation arises is set in advance as the reference rotation speed N1(e.g., 1000 rpm) and the reference rotation speed N1 is compared withthe actual rotation speed N in step S1.

If it is decided in step S1 that the motor rotation speed N is higherthan the reference rotation speed N1, the operation proceeds to step S2to ascertain a flag value. The flag is set to 0 in the initial state andis set to 1 once the motor rotation speed N exceeds the referencerotation speed N1. If it is decided in step S2 that the flag is set to0, the operation proceeds to step S3, whereas the operation makes areturn if the flag is determined to be set to 1. In step S3, a decisionis made as to whether or not a front step operation has been performedat the accelerator pedal 22 based upon a signal provided by the pressureswitch 41A. If an affirmative decision is made in step S3, the operationproceeds to step S4 to output a control signal to a solenoid of thesolenoid controlled directional control valve 25B so as to switch thesolenoid controlled directional control valve 25B to the position “b”.Subsequently, the flag is set to 1 in step S5, before the operationmakes a return.

If, on the other hand, a negative decision is made in step S3, theoperation proceeds to step S6 to make a decision as to whether or not arear step operation has been performed at the accelerator pedal 22 basedupon a signal provided by the pressure switch 41B. If an affirmativedecision is made in step S6, the operation proceeds to step S7, whereasthe operation makes a return if a negative decision is made in step S6.In step S7, a control signal is output to a solenoid at the solenoidcontrolled directional control valve 25A to switch the solenoidcontrolled directional control valve 25A to the position “b”, before theoperation proceeds to step S5.

If it is decided in step S1 that the motor rotation speed N is equal toor lower than the reference rotation speed N1, the operation proceeds tostep S8. In step S8, a control signal is output to the solenoids of boththe solenoid controlled directional control valves 25A and 25B to switchthe solenoid controlled directional control valves 25A and 25B to theposition “a”. Next, the flag is set to 0 in step S9, before theoperation makes a return.

The operation that characterizes the first embodiment adopting thestructure described above is now explained.

As the neutral switch 43 is turned on, the solenoid controlleddirectional control valves 25A and 25B are both switched to the position“b”, thereby cutting off the supply of pilot pressure to the controlvalve 12. In this state, the pressure oil from the main pump 11 is notguided to the traveling motor 5 even if the accelerator pedal 22 isoperated and the vehicle cannot travel along either the forwarddirection or the reverse direction.

As the neutral switch 43 is turned off, the solenoid controlleddirectional control valves 25A and 25B are switched in response to thecontrol signal provided by the controller 40. If the vehicle is in astationary state, the motor rotation speed N is 0, and accordingly, thesolenoid controlled directional control valves 25A and 25B are switchedto the position “a” and the flag is set to 0 (steps S8 and S9). If thetransmission 7 is shifted to low or high and a front step operation isperformed at the accelerator pedal 22 in this state, the control valve12 is switched to the F position and the pressure oil from the main pump11 is guided to the B port-side pipeline. Drive of the traveling motor 5thus starts and the vehicle starts traveling forward.

When the rotation speed of the traveling motor 5 exceeds the referencerotation speed N1, the solenoid controlled directional control valve 25Bis switched to the position “b” and the flag is set to 1 (steps S4 andS5). As a result, the pilot port at the control valve 12 comes intocommunication with the reservoir via the solenoid controlled directionalcontrol valve 25B. If a reverse operation (rear step operation) isperformed at the accelerator pedal 22 in this state, the pressure oilfrom the pilot pump 21 becomes cut off by the solenoid controlleddirectional control valve 25B, stopping the supply of the pilot pressureto the control valve 12 and thus switching the control valve 12 to the Nposition. Subsequently, a braking pressure is generated in the Aport-side pipeline with the counterbalance valve 13 in a manner similarto that with which a braking pressure is generated when an operation ofthe accelerator pedal 22 ceases, and while the vehicle travels forwardwith an inertial force, a hydraulic braking force is applied so that themotor rotation speed slows down. As a result, the motor drive pressurebecomes reduced as in a normal deceleration operation, the drivepressure guided to the pistons 17 and 18 from the shuttle valve 16 isreduced and thus, the motor displacement is not allowed to increase.Consequently, the extent to which the quantity of the required oilintake at the motor increases is minimized and the occurrence ofcavitation is prevented. This state is sustained until the motorrotation speed N becomes equal to or lower than the reference rotationspeed N1.

Once the motor rotation speed becomes equal to or lower than thereference rotation speed N1, the solenoid controlled directional controlvalves 25A and 25B are each switched to the position “a” (step S8).Consequently, the pilot pressure is applied to the control valve 12,switching the control valve 12 to the R position to guide the pressureoil from the main pump 11 into the A port-side pipeline. This results inan increase in the drive pressure guided to the pistons 17 and 18,which, in turn, increases the motor displacement. While the motordisplacement increases in response to the reverse operation at theaccelerator pedal 22 in this situation, the motor rotation speed N islow and for this reason, the traveling motor 5 does not require a largequantity of oil intake and any shortage in the quantity of oil intakecan be fully supplemented with the oil supplied from the makeup port 19.

By allowing the motor displacement to increase in response to thereverse operation at the accelerator pedal 22 when the motor rotationspeed is equal to or lower than the reference rotation speed N1 asdescribed above, it becomes possible to engage the traveling motor 5 inreverse rotation with a high torque immediately after stopping thetraveling motor 5. The vehicle traveling direction can be switched moreefficiently by allowing a motor displacement increase in this mannerrather than by performing a rear step operation at the accelerator pedal22 after stopping the traveling motor 5.

It is to be noted that the motor displacement undergoes a similar changeif a reverse or opposite operation (front step operation) is performedat the accelerator pedal 2 while the vehicle is traveling in response toa rear step operation at the accelerator pedal 22.

As described above, the traveling pilot circuit achieved in the firstembodiment includes the solenoid controlled directional control valves25A and 25B and if the rotation speed of the traveling motor 5 isgreater than the reference rotation speed N1, generation of a travelingpilot pressure in response to a reverse operation at the acceleratorpedal 22 is prohibited. As a result, the control valve 12 is switched tothe neutral position, preventing an increase in the motor displacementand thus preventing the occurrence of cavitation. In addition, when therotation speed of the traveling motor 5 is equal to or lower than thereference rotation speed N1, the generation of the traveling pilotpressure in response to a reverse operation at the accelerator pedal 22is allowed and thus, the forward and reverse traveling directions can beswitched efficiently at low speed. As the solenoid controlleddirectional control valves 25A and 25B are installed in the travelingpilot circuit, they only need to be capable of withstanding low pressureand the hydraulic circuit can be provided at low cost. Since the controlvalve 12 is switched to the neutral position in response to a reverseoperation at the accelerator pedal 22, the oil discharged from the mainpump 11 is not guided to the drive circuit for the traveling motor 5 andfor this reason, no unnecessary load is applied to the pump 11. Sincethe neutral switch 43 that issues a command for a neutral travelingstate is included and the neutral state command overrides the control bythe controller 40 so as to disable switching of the solenoid controlleddirectional control valves 25A and 25B, a stable neutral traveling statecan be sustained.

Second Embodiment

A second embodiment of the present invention is now explained inreference to FIG. 5.

FIG. 5 is a circuit diagram of a traveling hydraulic circuit in thewheel hydraulic excavator achieved in the second embodiment. It is to benoted that the same reference numerals are assigned to componentsidentical to those in FIG. 2 and the following explanation focuses onthe differentiating features.

While a pair of solenoid controlled directional control valves 25A and25B are installed in the traveling pilot circuit in the firstembodiment, the second embodiment further includes a pair of solenoidcontrolled directional control valves 26A and 26B disposed between thecontrol valve 12 and the brake valve 14. By switching the solenoidcontrolled directional control valves 26A and 26B as described later,the flow of pressure oil from the main pump 11 to the brake valve 14 iseither allowed or disallowed while generation of a traveling pilotpressure itself is allowed, unlike in the first embodiment.

The solenoid controlled directional control valves 25A and 25B are notconnected to the controller 40 but are connected to the neutral switch43 alone to be switched in response to an operation of the neutralswitch 43. Namely, the solenoid controlled directional control valves25A and 25B are each switched to the position “b” in response to an ONoperation at the neutral switch 43 and are each switched to the position“a” in response to an OFF operation at the neutral switch.

The solenoid controlled directional control valves 26A and 26B areswitched in a manner similar to that explained in reference to the firstembodiment, through the processing executed in the controller 40.Namely, the solenoid controlled directional control valve 26B isswitched to the position “b” once the motor rotation speed N exceeds thereference rotation speed N1 in response to a front step operation at theaccelerator pedal 22, and the solenoid controlled directional controlvalve 26A is switched to the position “b” as the motor rotation speed Nexceeds the reference rotation speed N1 in response to a rear stepoperation at the accelerator pedal 22. As the motor rotation speed Nbecomes equal to or lower than the reference rotation speed N1 followinga reverse operation at the accelerator pedal 22, the solenoid controlleddirectional control valves 26A and 26B are individually switched to theposition “a”.

In the second embodiment, as a reverse operation is performed at theaccelerator pedal 22 by depressing on the rear side after the motorrotation speed N exceeds the reference rotation speed N1 in response toa front step operation at the accelerator pedal 22, the pilot pressureis applied to the control valve 12, switching the control valve from theposition F to the position R. Since the solenoid controlled directionalcontrol valve 26B is switched to the position “b” at this point, thepressure oil from the main pump 11 is not supplied to the brake valve14, allowing the counterbalance valve 13 to remain at the neutralposition, and the motor drive pressure becomes lowered as in a normaldeceleration operation, as explained earlier. As a result, the increasein the motor displacement is minimized to prevent the occurrence ofcavitation.

As the motor rotation speed N becomes equal to or lower than thereference rotation speed N1 following a reverse operation at theaccelerator pedal 22, the solenoid controlled directional control valves26A and 26B are switched to the position “a” and the pressure oil fromthe main pump 11 is guided to the brake valve 14. As a result, the motordrive pressure increases and the motor displacement, too, increases.Since the motor rotation speed N is low in this situation, the concernfor the occurrence of cavitation does not arise and the forwardtraveling direction and the reverse traveling direction can be switchedefficiently.

As described above, in the second embodiment, having the solenoidcontrolled directional control valves 26A and 26B disposed between thecontrol valve 12 and the brake valve 14, when the rotation speed of thetraveling motor 5 is higher than the reference rotation speed N1, thesupply of pressure oil to the brake valve 14 in response to a reverseoperation at the accelerator pedal 22 is prohibited and when thetraveling motor rotation speed is equal to or lower than the allowablereference rotation speed N1, the pressure oil supply is allowed. As aresult, the occurrence of cavitation can be prevented with a high degreeof effectiveness.

Third Embodiment

A third embodiment of the present invention is now explained inreference to FIGS. 6 through 8.

FIG. 6 is a circuit diagram of a traveling hydraulic circuit in thewheel hydraulic excavator achieved in the third embodiment. It is to benoted that the same reference numerals are assigned to componentsidentical to those in FIG. 2 and the following explanation focuses onthe differentiating features.

A traveling pilot circuit in the third embodiment differentiates it fromthe first embodiment. Namely, while a pair of pilot valves 23A and 23Band a pair of slow return valves 24A and 24B are provided and the pilotvalves 23A and 23B are individually driven in response to a front stepoperation and a rear step operation performed at the accelerator pedal22 in the first embodiment, a single pilot valve 23 and a single slowreturn valve 24 are utilized and the pilot valve 23 is driven inresponse to an operation at the accelerator pedal 22 in the thirdembodiment.

A forward/backward switching valve 27 is connected next to the slowreturn valve 24. As a solenoid 27F of the forward/backward switchingvalve 27 is excited, the forward/backward switching valve 27 is switchedto a position F, as a solenoid 27R of the forward/backward switchingvalve 27 is excited, the forward/backward switching valve 27 is switchedto a position R, and as the solenoids 27F and 27R become demagnetized,the forward/backward switching valve is switched to a position N. If theforward/backward switching valve 27 is switched to the position F or theposition R while the accelerator pedal 22 is being depressed, a pilotpressure is applied to a pilot port at the control valve 22, therebyswitching the control valve 12 to the F position or the R position. Ifthe forward/backward switching valve 27 is switched to the position N,no pilot pressure is applied to the control valve 12 and the controlvalve 12 is switched to the N position.

FIG. 7 is a block diagram of the control circuit that controls theforward/backward switching valve 27. It is to be noted that the samereference numerals are assigned to components identical to those in FIG.3. The rotation speed sensor 42 and a forward/backward selector switch51 are connected to a controller 50. The forward/backward selectorswitch 51 is installed in the operator's cab 3, and can be operated toan F position, an N position or an R position to output a command forthe vehicle to travel forward, to travel backward or to assume a neutralstate. An F contact point of the forward/backward selector switch isconnected to the solenoid 27F of the forward/backward switching valve 27via a relay 52, whereas an R contact point of the forward/backwardselector switch is connected to the solenoid 27 R via a relay 53. Thecontroller 50 executes processing described below and outputs a controlsignal to coils at the relays 52 and 53.

FIG. 8 presents a flowchart of an example of processing that may beexecuted in the controller 50. It is to be noted that the same stepnumbers are assigned to steps identical to those in FIG. 4 and thefollowing explanation focuses on the differentiating features. Afterdeciding in step S2 that the flag is set to 0, the operation proceeds tostep S14 to make a decision as to whether or not the forward/backwardselector switch 51 is currently set to the F position. If an affirmativedecision is made in step S14, the operation proceeds to step S15 tosupply power to the coil at the relay 53. In response, the relay 53 isswitched to a contact point “b”, thereby preventing the solenoid 27Rfrom becoming excited. If, on the other hand, a negative decision ismade in step S14, the operation proceeds to step S17 to make a decisionas to whether or not the forward/backward selector switch 51 iscurrently set to the R position. If an affirmative decision is made instep S17, the operation proceeds to step S18 to supply power to the coilat the relay 52. In response, the relay 52 is switched to a contactpoint “b” and the solenoid 27F is thus prevented from becoming excited.If the switch 51 is at the N position, a negative decision is made instep S17 and the operation makes a return.

The operation proceeds to step S19 after making a decision in step S1that the motor rotation speed N is equal to or lower than the referencerotation speed N1. In step S19, the power supply to the coils at therelays 52 and 53 is stopped. In response, the relays 52 and 53 are bothswitched to the contact points “a”.

If the forward/backward selector switch 51 is operated to the F positionwhile the vehicle is, for instance, in a stationary state and theaccelerator pedal 22 is depressed in the third embodiment describedabove, the relays 52 and 53 are switched to the position “a” and thesolenoid 27F becomes excited (step S19). As a result, theforward/backward switching valve 27 is switched to the position F, thepilot pressure is applied to the control valve 12 and the control valve12, in turn, is switched to the F position. With the control valve thusswitched, the pressure oil from the main pump 11 is guided to thetraveling motor 5 and the vehicle starts traveling forward.

Once the rotation speed of the traveling motor 5 exceeds the referencerotation speed N1, the relay 53 is switched to the position “b” and thepower supply to the solenoid 27 R stops (step S15). In this state, thesolenoid 27R does not become excited even if the forward/backwardselector switch 51 is operated to the R position, i.e., even if areverse operation is performed at the switch 51, and theforward/backward switching valve 27 is switched to the position N. Thegeneration of a traveling pilot pressure is thus prevented, the controlvalve 12 is switched to the neutral position and the counterbalancevalve 13, too, is switched to the neutral position, resulting in areduction in the motor drive pressure due to the normal brakingfunction, which disallows an increase in the motor displacement.

As explained earlier, once the rotation speed of the traveling motor 5becomes equal to or lower than the reference rotation speed N1 after theforward/backward selector switch 51 is operated to the R position, therelay 53 is switched to the position “a” and the solenoid 27R becomesexcited (step S19). In response, the forward/backward switching valve 27is switched to the position R and the control valve 12 is also switchedto the position R. As a result, the motor drive pressure increases toincrease the motor displacement. However, since the motor rotation speedN is low, cavitation does not occur.

As described above, the traveling pilot circuit in the third embodimentincludes the forward/backward switching valve 27 that can be switchedthrough a switching operation and when the rotation speed of thetraveling motor 5 is higher than the reference rotation speed N1, achangeover at the forward/backward switching valve 27 in response to areverse operation at the switch 51 is prohibited so as to prevent thegeneration of a traveling pilot pressure. Consequently, since the motordisplacement is not allowed to increase, the occurrence of cavitation isprevented.

Fourth Embodiment

A fourth embodiment of the present invention is now explained inreference to FIGS. 9 through 11.

FIG. 9 is a circuit diagram of a traveling hydraulic circuit in thewheel hydraulic excavator achieved in the fourth embodiment. It is to benoted that the same reference numerals are assigned to componentsidentical to those in FIG. 2 and the following explanation focuses onthe differentiating features

While a pair of solenoid controlled directional control valves 25A and25B are installed in the traveling pilot pipeline in the firstembodiment, the fourth embodiment further includes an solenoidcontrolled directional control valve 28 installed in a drive pressuresupplied pipeline extending from the shuttle valve 16 to the controlpiston 17 and the servo piston 18. As shown in FIG. 9, as the solenoidcontrolled directional control valve 28 is switched to the position “a”,a drive pressure is guided to the pistons 17 and 18, and the motordisplacement assumes a value corresponding to the drive pressure. As thesolenoid controlled directional control valve 28 is switched to aposition “b”, the supply of drive pressure from the shuttle valve 16 tothe pistons 17 and 18 stops, thereby setting the motor displacement tothe minimum value. It is to be noted that the solenoid controlleddirectional control valves 25A and 25B are not connected to a controller60 and are switched in response to an operation of the neutral switch 43as in the second embodiment.

FIG. 10 is a block diagram of a control circuit that controls thesolenoid controlled directional control valve 28. It is to be noted thatthe same reference numerals are assigned to components identical tothose in FIG. 3. The rotation speed sensor 42 and the pressure switches41A and 41B are connected to the controller 60. The controller 60executes the following processing in response to signals input theretofrom the rotation speed sensor and the pressure switches and outputs acontrol signal to a solenoid at the solenoid controlled directionalcontrol valve 28.

FIG. 11 presents a flowchart of an example of processing that may beexecuted in the controller 60. It is to be noted that the same stepnumbers are assigned to steps identical to those in FIG. 4, and thefollowing explanation focuses on the differentiating features. Theoperation proceeds to step S21 after making a negative decision in stepS1, and the value of an F flag is ascertained in step S21. The F flag isset to 1 (step S24) if the motor rotation speed exceeds the referencerotation speed N1 following a front step operation at the acceleratorpedal 22. If the value of the F flag is judged to be 0 in step S21, theoperation proceeds to step S22 to ascertain the value of an R flag. TheR flag is set to 1 (step S26) if the motor rotation speed exceeds thereference rotation speed N1 following a rear step operation at theaccelerator pedal 22. If the value of the R flag is judged to be 0 instep S22, the operation proceeds to step S23.

In step S23, a decision is made based upon a signal provided from thepressure switch 41A as to whether or not a front step operation has beenperformed at the accelerator pedal 22. If an affirmative decision ismade in step S23, the operation proceeds to step S24 to set the F flagto 1, before the operation makes a return. If a negative decision ismade in step S23, the operation proceeds to step S25 to make a decisionbased upon a signal provided from the pressure switch 41B as to whetheror not a rear step operation has been performed at the accelerator pedal22. The operation proceeds to step S26 if an affirmative decision ismade in step S25, whereas the operation makes a return if a negativedecision is made in step S25. In step S26, the R flag is set to 1, andthen the operation makes a return.

If it is decided in step S21 that the F flag is set to 1, the operationproceeds to step S27 to make a decision based upon a signal providedfrom the pressures witch 41B as to whether or not a rear step operationhas been performed at the accelerator pedal 22. If an affirmativedecision is made in step S27, the operation proceeds to step S29 tooutput a control signal to the solenoid at the solenoid controlleddirectional control valve 28 and switch the solenoid controlleddirectional control valve 28 to the position “b”. If a negative decisionis made in step S27, the operation proceeds to step S30 to output acontrol signal to the solenoid at the solenoid controlled directionalcontrol valve 28 and switch the solenoid controlled directional controlvalve 28 to the position “a”.

If, on the other hand, it is decided in step S22 that the R flag is setto 1, the operation proceeds to step S28 to make a decision based upon asignal provided from the pressure switch 41A as to whether or not afront step operation has been performed at the accelerator pedal 22. Theoperation proceeds to step S29 if an affirmative decision is made instep S28, whereas the operation proceeds to step S30 if a negativedecision is made in step S28.

If it is decided in step S1 that the motor rotation speed N is equal toor lower than the reference rotation speed N1, the operation proceeds tostep S31. In step S31, a control signal is output to the solenoid at thesolenoid controlled directional control valve 28, thereby switching thesolenoid controlled directional control valve 28 to the position “a”.Then, the F flag is set to 0 in step S32 and the R flag is set to 0 instep S33.

If a front step operation, for instance, is performed at the acceleratorpedal 22 in the fourth embodiment adopting the structure describedabove, the control valve 12 is switched to the F position and thepressure oil from the main pump 11 causes the traveling motor 5 torotate. At this time, the solenoid controlled directional control valve28 is switched to the position “a” through the processing explainedearlier (step S31), the drive pressure is guided to the pistons 17 and18, and the motor displacement achieves a value corresponding to thedrive pressure.

While the F flag is set to 1 (step S24) once the motor rotation speedexceeds the reference rotation speed N1, the solenoid controlleddirectional control valve 28 remains at the position “a” (step S30) aslong as a reverse operation is not performed at the accelerator pedal22. If a reverse operation is performed at the accelerator pedal 22 inthis state, the solenoid controlled directional control valve 28 isswitched to the position “b” (step S29). The supply of the drivepressure to the pistons 17 and 18 is thus cut off, and as the motordisplacement is set to the minimum value, the occurrence of cavitationis prevented.

If the motor rotation speed becomes equal to or lower than the referencerotation speed N1 following a reverse operation at the accelerator pedal22, the solenoid controlled directional control valve 28 is switched tothe position “a” (step S31). In response, the drive pressure is suppliedto the pistons 17 and 18, which increases the motor displacement.

As described above, in the fourth embodiment which includes the solenoidcontrolled directional control valve 28 installed in the drive pressuresupply pipeline extending from the shuttle valve 16 to the pistons 17and 18, an increase in the motor displacement is disallowed byprohibiting the supply of the drive pressure to the pistons 17 and 18 ifa reverse operation is performed at the accelerator pedal 22 in a statewhere the rotation speed of the traveling motor 5 is higher than thereference rotation speed N1. As a result, the occurrence of cavitationcan be prevented.

It is to be noted that while an increase in the motor displacement isallowed or prohibited in correspondence to the rotation speed of thetraveling motor 5 in the explanation given above, the inertial force ofthe vehicle also has a correlation with the grade of the road surfaceand the vehicle weight as well as the motor rotation speed. Accordingly,it is desirable to set the reference rotation speed N1 by taking thesefactors into consideration as well in order to reliably prevent theoccurrence of cavitation.

When the grade of the road surface is to be taken into consideration, aninclination sensor, for instance, may be mounted at the vehicle todetect the grade of the road surface and the reference rotation speed N1should be set to a smaller value as the angle of inclination becomeslarger, i.e., as the inertial force increases as shown in FIG. 12.

When the vehicle weight is to be taken into consideration, a targetrotation speed NA for the traveling motor 5 corresponding to the angleof inclination of a downhill slope should be calculated based upon apredetermined relationship such as that shown in FIG. 13( a). Then,based upon a relationship such as that shown in FIG. 13( b), the vehicleweight is judged to be greater as the difference between the targetrotation speed NA and the actual rotation speed N becomes greater andthe reference rotation speed N1 should be set to a smaller value for aheavier vehicle.

Instead of detecting the rotation speed of the traveling motor 5, aphysical quantity having a correlation to the motor rotation speed maybe detected. For instance, the rotation speed of the output shaft of thetransmission 7 may be detected and the solenoid controlled directionalcontrol valves 25A, 25B, 26A, 26B and 28 and the relays 52 and 56 may beswitched depending upon whether or not the detected value exceeds areference rotation speed N1. In this case, the reference rotation speedN1 for the output shaft should be set in correspondence to the gearratio at the transmission. Namely, the reference rotation speed shouldbe set to a smaller value when the gear is shifted to low (the gearratio is large) compared to when the gear is shifted to high (the gearratio is small).

While a forward travel command or a reverse travel command is issuedthrough a front step operation or a rear step operation at theaccelerator pedal 22 or through operations performed at the acceleratorpedal 22 and the forward/backward selector switch 51, these commands mayinstead be issued via another operating member (e.g., a lever).

While a reverse operation at the accelerator pedal 22 is detected withthe pressure switches 41A and 41B or the forward/backward selectorswitch 51, a reverse operation at the accelerator pedal 22 may bedetected with a limit switch or the like instead.

While the solenoid controlled directional control valves 25A and 25B andthe like are switched on/off once the motor rotation speed exceeds thereference rotation speed N1 in the embodiments described above, they mayinstead be switched incrementally in correspondence to the motorrotation speed.

While the self pressure displacement control mechanism of the travelingmotor 5 holds the motor displacement at the minimum displacement levelover a predetermined relatively low motor drive pressure range, themotor displacement may instead be adjusted in correspondence to themotor drive pressure without setting any such predetermined range.

INDUSTRIAL APPLICABILITY

While an explanation is given above on an example in which the presentinvention is adopted in a wheel hydraulic excavator, the presentinvention may also be adopted in other types of hydraulically drivenvehicles including construction machines such as wheel loaders and truckcranes.

1. A hydraulically driven vehicle, comprising: a hydraulic pump drivenby an engine; a variable displacement hydraulic motor for travelingdriven by pressure oil from the hydraulic pump; a motor displacementcontrol device that adjusts a displacement of the hydraulic motor incorrespondence to a drive pressure at the hydraulic motor; an operatingmotor through which a forward travel command and a backward travelcommand for the vehicle and a command for acceleration and decelerationfor the vehicle are issued; a pilot valve configured to provide a pilotpressure oil in accordance with an extent to which the operating memberis operated; a control valve configured to be driven by the pilotpressure oil from the pilot valve to control a flow of pressure oil fromthe hydraulic pump to the hydraulic motor; a counterbalance valve, thatis disposed between the control valve and the hydraulic motor,configured to be switched to an opening position by the pressure oilsupplied from the hydraulic pump so as to open a return path for oil toreturn from the hydraulic motor to the control valve, and to be switchedto a closing position so as to close the return path as the pressure oilfrom the hydraulic pump is stopped; a directional control valve thatcontrols a flow of the pilot pressure oil which acts on the controlvalve; a neutral command device that issues a command for switching thecontrol valve to a neutral position; a reverse operation detectiondevice that detects a reverse operation of the operating memberperformed to a reverse side opposite from a direction along which thevehicle is advancing; a rotation speed detection device that detects aphysical quantity having a correlation to a rotation speed of thehydraulic motor; and a control device that stops supply of the pressureoil from the hydraulic pump to the hydraulic motor by interrupting theflow of the pilot pressure oil to the control valve in response to thecommand issued from the neutral command device, regardless of operationof the operating member, wherein: when the command is not issued fromthe neutral command device, the control device (a) stops supply of thepressure oil from the hydraulic pump to the hydraulic motor byinterrupting the flow of the pilot pressure oil to the control valve ifthe physical quantity detected by the rotation speed detection deviceexceeds a reference value and the reverse operation at the operatingmember is detected by the reverse operation detection device, and (b)allows the pilot pressure oil to flow to the control valve, even if thereverse operation at the operating member is detected, if the physicalquantity detected by the rotation speed detection device is equal to orsmaller than the reference value.
 2. A hydraulically driven vehicle,comprising: a hydraulic pump driven by an engine; a variabledisplacement hydraulic motor for traveling driven by pressure oil fromthe hydraulic pump; a motor displacement control device that adjusts adisplacement of the hydraulic motor in correspondence to a drivepressure at the hydraulic motor; a forward/backward operating memberthrough which a forward travel command, a backward travel command and aneutral command for the vehicle are issued; a traveling operation memberthrough which a command for acceleration and deceleration for thevehicle is issued in accordance with an extent to which the travelingoperation member is operated; a pilot valve configured to provide apilot pressure oil in response to an operation of the travelingoperation member; a control valve configured to be driven by the pilotpressure oil from the pilot valve to control a flow of pressure oil fromthe hydraulic pump to the hydraulic motor; a counterbalance valve, thatis disposed between the control valve and the hydraulic motor,configured to be switched to an opening position by the pressure oilsupplied from the hydraulic pump so as to open a return path for oil toreturn from the hydraulic motor to the control valve, and to be switchedto a closing position so as to close the return path as the pressure oilfrom the hydraulic pump is stopped; a directional control valve thatcontrols a flow of the pilot pressure oil which acts on the controlvalve in response to an operation of the forward/backward operatingmember; a reverse operation detection device that detects a reverseoperation of the forward/backward operating member performed to areverse side opposite from a direction along which the vehicle isadvancing; a rotation speed detection device that detects a physicalquantity having a correlation to a rotation speed of the hydraulicmotor; and a control device that stops supply of the pressure oil fromthe hydraulic pump to the hydraulic motor by interrupting the flow ofthe pilot pressure oil to the control valve in response to the neutralcommand issued from the forward/backward operating member, regardless ofthe operation of the traveling operation member, wherein: when theneutral command is not issued from the forward/backward operatingmember, the control device (a) stops supply of the pressure oil from thehydraulic pump to the hydraulic motor by interrupting the flow of thepilot pressure oil to the control valve if the physical quantitydetected by the rotation speed detection device exceeds a referencevalue and the reverse operation at the forward/backward operating memberis detected by the reverse operation detection device, and (b) allowsthe pilot pressure oil to flow to the control valve, even if the reverseoperation at the forward/backward operating member is detected, if thephysical quantity detected by the rotation speed detection device isequal to or smaller than the reference value.
 3. A hydraulically drivenvehicle, comprising: a hydraulic pump driven by an engine; a variabledisplacement hydraulic motor for traveling driven by pressure oil fromthe hydraulic pump; a motor displacement control device that adjusts adisplacement of the hydraulic motor in correspondence to a drivepressure at the hydraulic motor; an operating member through which aforward travel command and a backward travel command for the vehicle anda command for acceleration and deceleration for the vehicle are issued;a pilot valve configured to provide a pilot pressure oil in accordancewith an extent to which the operating member is operated; a controlvalve configured to be driven by the pilot pressure oil from the pilotvalve to control a flow of pressure oil from the hydraulic pump to thehydraulic motor; a counterbalance valve, that is disposed between thecontrol valve and the hydraulic motor, configured to be switched to anopening position by the pressure oil supplied from the hydraulic pump soas to open a return path for oil to return from the hydraulic motor tothe control valve, and to be switched to a closing position so as toclose the return path as the pressure oil from the hydraulic pump isstopped; a first directional control valve that is disposed between thecontrol valve and the counterbalance valve and controls a flow of thepressure oil from the hydraulic pump to the hydraulic motor; a neutralcommand device that issues a command for switching the control valve toa neutral position; a second directional control valve configured to beswitched to interrupt the pilot pressure oil which acts on the controlvalve in response to the command issued from the neutral command device,regardless of operation of the operating member, and configured to beswitched to allow the pilot pressure to flow to the control valve whenthe command is not issued from the neutral command device; a reverseoperation detection device that detects a reverse operation of theoperating member performed to a reverse side opposite from a directionalong which the vehicle is advancing; a rotation speed detection devicethat detects a physical quantity having a correlation to a rotationspeed of the hydraulic motor; and a control device that controls thefirst directional control valve such that the first directional controlvalve (a) stops the flow of the pressure oil from the hydraulic pump tothe hydraulic motor when the physical quantity detected by the rotationspeed detection device exceeds a reference value and the reverseoperation at the operating member is detected by the reverse operationdetection device, and (b) allows the pilot pressure oil to flow to thecontrol valve, even if the reverse operation at the operating member isdetected, when the physical quantity detected by the rotation speeddetection device is equal to or smaller than the reference value.
 4. Ahydraulically driven vehicle according to claim 1, wherein: the physicalquantity is a vehicle speed and the reference value is set to a smallervalue as a gear ratio increases.
 5. A hydraulically driven vehicleaccording to claim 1, further comprising: an inertial force detectiondevice that detects an inertial force applied to the vehicle, wherein:the reference value is set to a smaller value as a greater inertialforce is detected.
 6. A hydraulically driven vehicle according to claim5, wherein: the inertial force detection device detects a grade of aroad surface and the reference value is set to a smaller value as thegrade becomes steeper.
 7. A hydraulically driven vehicle according toclaim 5, wherein: the inertial force detection device detects a vehicleweight, and the reference value is set to a smaller value as the vehicleweight becomes greater.
 8. A hydraulically driven vehicle according toclaim 2, wherein: the physical quantity is a vehicle speed and thereference value is set to a smaller value as a gear ratio increases. 9.A hydraulically driven vehicle according to claim 2, further comprising:an inertial force detection device that detects an inertial forceapplied to the vehicle, wherein: the reference value is set to a smallervalue as a greater inertial force is detected.
 10. A hydraulicallydriven vehicle according to claim 9, wherein: the inertial forcedetection device detects a grade of a road surface and the referencevalue is set to a smaller value as the grade becomes steeper.
 11. Ahydraulically driven vehicle according to claim 9, wherein: the inertialforce detection device detects a vehicle weight, and the reference valueis set to a smaller value as the vehicle weight becomes greater.
 12. Ahydraulically driven vehicle according to claim 3, wherein: the physicalquantity is a vehicle speed and the reference value is set to a smallervalue as a gear ratio increases.
 13. A hydraulically driven vehicleaccording to claim 3, further comprising: an inertial force detectiondevice that detects an inertial force applied to the vehicle, wherein:the reference value is set to a smaller value as a greater inertialforce is detected.
 14. A hydraulically driven vehicle according to claim13, wherein: the inertial force detection device detects a grade of aroad surface and the reference value is set to a smaller value as thegrade becomes steeper.
 15. A hydraulically driven vehicle according toclaim 13, wherein: the inertial force detection device detects a vehicleweight, and the reference value is set to a smaller value as the vehicleweight becomes greater.